| Glauconite | |
|---|---|
| General | |
| Category | Phyllosilicate |
| Chemical formula | (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2 |
| Crystal symmetry | Monoclinic prismatic 2/m |
| Unit cell | a = 5.234 Å, b = 9.066 Å, c = 10.16 Å; β = 100.5°; Z = 2 |
| Identification | |
| Color | Blue green, green, yellow green |
| Crystal habit | Platy micaceous or rounded pellets |
| Crystal system | Monoclinic |
| Cleavage | Perfect [001] |
| Mohs scale hardness | 2 |
| Luster | Dull - earthy |
| Streak | Light green |
| Diaphaneity | Translucent to nearly opaque. |
| Specific gravity | 2.4 - 2.95 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.590 - 1.612 nβ = 1.609 - 1.643 nγ = 1.610 - 1.644 |
| Birefringence | δ = 0.020 - 0.032 |
| Pleochroism | X = yellow-green, green; Y = Z = deeper yellow, bluish green |
| References | [1][2][3] |
Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color with very low weathering resistance and very friable.[4]
It crystallizes with a monoclinic geometry. Its name is derived from the Greek glaucos (γλαυκος) meaning 'gleaming' or 'silvery', to describe the appearance of the blue-green color, presumably relating to the sheen and blue-green color of the sea's surface. Its color ranges from olive green, black green to bluish green. In the Mohs scale it has hardness of 2. The relative specific gravity range is 2.4 - 2.95. It is normally found in dark green rounded pellets with the dimension of a sand grain size. It can be confused with chlorite (also of green color) or with some other clay minerals.
Normally, glauconite is considered a diagnostic mineral indicative of continental shelf marine depositional environments with slow rates of accumulation. For instance, it appears in Jurassic/lower Cretaceous deposits of greensand, so-called after the coloration caused by glauconite. It can also be found in sand or clay formations, or in impure limestones and in chalk. It develops as a consequence of diagenetic alteration of sedimentary deposits, bio-chemical reduction and subsequent mineralogical changes affecting iron-bearing micas such as biotite, and is also influenced by the decaying process of organic matter degraded by bacteria in marine animal shells. Glauconite forms under reducing conditions in sediments and such deposits are commonly found in nearshore sands, open oceans and the Mediterranean Sea, but not in the Black Sea or in fresh-water lakes.
The wide distribution of these sandy deposits was first made known by naturalists on board the fifth HMS Challenger, in the expedition of 1872-1876.
Glauconite has long been used in Europe as a pigmentation agent for artistic oil paint, especially in Russian "icon paintings". It is also found as mineral pigment in wall paintings from the ancient Roman Gaul.[5]
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